The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which include an internal gear set and a pair of relatively movable valve members operable to communicate fluid to and from the gear set.
Although it should become apparent from the subsequent description of the invention that it may be useful with many types and configurations of rotary fluid pressure devices, including both pumps and motors, it is especially advantageous when used in a fluid motor, and will be described in connection therewith.
Also, although the invention may be used with devices having various types of internal gear sets, the invention is especially adapted for use in a device wherein the internal gear set comprises a gerotor displacement mechanism.
Fluid motors of the type utilizing a gerotor displacement mechanism to convert fluid pressure into a rotary output are especially suited for low speed, high torque applications. Typically, in fluid motors of this type, there are two relatively movable valve members. One of the valve members is stationary and provides a fluid passage communicating with each of the volume chambers of the gerotor, while the other valve member rotates, relative to the stationary valve member, at the speed of rotation of the rotatable member of the gerotor set. Valving of the type described is referred to as being "low speed" to distinguish it from the type of valving referred to as "high speed", wherein the rotatable valve member rotates at the orbiting speed of the orbiting member of the gerotor set.
One of the problems associated with fluid motors of the type described has been the operating noise level. A certain amount of noise is inevitable in such a device, but it is now believed that a noticeable amount of the noise is caused by the "trapping" of fluid in a volume chamber as it reaches its minimum volume position, and/or the "cavitation" which occurs in the volume chamber just after it passes the minimum volume position. Both of these phenomena will be described in somewhat greater detail subsequently, as they relate to fluid motors of the type to which the invention relates.
It is known in the prior art to attempt to overcome the problems of trapping and cavitation by intentionally introducing deviations from the theoretical valve timing. For example, it is known to permit the mating valve passages to remain in fluid communication for a longer time, to minimize the trapping of fluid in the volume chamber. However, this type of approach is generally considered undesirable because it substantially increases the opportunity for cross port leakage to occur, i.e., the establishment of a leakage path between the high pressure inlet fluid and the low pressure exhaust fluid. When cross port leakage occurs, the volumetric efficiency of the fluid motor is decreased.